Optical information media include read only optical discs as typified by compact discs, erasable optical recording discs such as magneto-optical recording discs and phase change type recording discs, and write-once optical recording discs using organic dyes as the recording material.
In general, optical information media have a high information density as compared with magnetic recording media. It is now required to further increase the information density for processing a very large quantity of information as in picture processing. Information density per unit area can be increased in two ways, by reducing a track pitch and by reducing the distance between record marks or phase pits for achieving an increased linear density. However, as the track density or linear density is increased relative to a beam spot of reading light, C/N of read signals becomes poor and eventually signal read-out becomes impossible. Resolution upon signal read-out is determined by the diameter of a beam spot. More particularly, when signals are read out through an optical system having an objective lens with a numerical aperture NA using reading light having a wavelength 1, a spatial frequency 2NA/1 becomes a limit of read-out. Therefore, for improving the C/N and resolution of read signals, it is effective to reduce the wavelength of reading light and to increase NA. Many research works have been carried out in this regard, but there still remain many technical problems to be solved.
JP-A 96926/1990 or U.S. Pat. No. 5,153,873 proposes a recording carrier having a layer of non-linear optical material for achieving super-resolution. This non-linear optical material changes its optical properties by incident radiation. Such changes include changes of transmittance, reflectance, and refractive index as well as deformation of the layer. When a high intensity beam is irradiated to the information-carrying surface through the non-linear optical material layer, smaller areas of the object can be read out by these optical changes.
The above-cited patent reference discloses a bleaching layer as one example of the non-linear optical material layer. The bleaching layer increases transmittance with the increasing intensity of incident radiation. Exemplary materials used in the bleaching layer are gallium arsenide, indium arsenide and indium antimony. However, since the layer of such non-linear optical material requires the absorption center to be entirely excited, reading light must have a high energy density, imposing difficulty to material and medium design.
The above-cited patent reference also discloses use of a phase change material as the non-linear optical material. Exemplary phase change materials are GaSb and InSb. The patent reference describes: "It has been found that the complex refractive index of this type of material is temperature dependent to such an extent that, even in the case of irradiation with an intensity remaining below the level at which the conversion from amorphous to crystalline or conversely occurs, there is a sufficiently large variation of the complex refractive index to enable layers of these materials to be used as non-linear layers in the sense of the present invention." Although the reason why such a change of complex refractive index occurs is not described in the patent reference, it is presumed that this change of complex refractive index involves a crystal-to-crystal transition of the non-linear optical material layer. In this case, since there is no need to melt the non-linear optical material, reading light of low power can be used. However, GaSb has a crystal-to-crystal transition temperature at a low temperature of about 30.degree. C. or a high temperature of about 590.degree. C. and InSb has a crystal-to-crystal transition temperature at a low temperature of about 150.degree. C. or a high temperature of about 500.degree. C. When the higher transition temperature of these phase change materials is utilized, reading light of high power must be used, giving rise to problems as mentioned above. On the other hand, when the lower transition temperature of these phase change materials is utilized, reading light of low power can be used. However, stable read-out is substantially impossible with GaSb because the transition temperature is extremely low. Problems also arise with InSb. Since the transition temperature is relatively low, the non-linear optical material layer is slow in cooling rate. Heat accumulates in the proximity of a mask layer to enlarge the apparent diameter of a beam spot, adversely affecting super-resolution read-out.
JP-A 89511/1993, 109117/1993, and 109119/1993 disclose an optical disc comprising a transparent substrate having formed therein phase pits which can be optically read out and a material layer thereon which changes its reflectance with temperature. This material layer is provided for achieving higher resolution beyond the limit determined by the reading light wavelength 1 and objective lens numerical aperture NA through approximately the same function as the non-linear optical material layer of JP-A 96926/1990. However, this material layer requires reading light of higher power because a crystal-to-liquid or amorphous-to-liquid change is necessary for read-out.